1. Field of the Invention
The present invention relates to a printed wiring board with a through hole which is blinded by an insulation resin film. In particular, the present invention relates to a printed wiring board with a through hole in which an insulation film is formed in order to blind thereof and to a process for producing such printed wiring board, thereby when electrical check that breaking of circuit patterns ( wires ) or resistance of circuit patterns on the printed wiring board is done by an electrical checker while fixing the printed wiring board on the checker or a fixing table thereof by sucking through a suction device, not only it can be done electrical check by inserting a top of a checker pin of the checker in the through hole, but also it can reliably prevent the insulation film from being dropped out from the through hole.
2. Description of Related Art
In general, after a printed wiring board is produced and before the printed wiring board is forwarded as a product, electrical check for checking whether circuit patterns are broken or resistance values of circuit patterns exist in a range of predetermined designed resistance values is conventionally conducted. Such electrical check of the printed wiring board is done by a check device having a checker pin, the check device being called "checker".
Here, there exist various checkers and recently a checker having a suction device through which the printed wiring board is sucked and fixed onto a predetermined position of the checker or a table thereof with a plurality of suction openings, is widely utilized. In such checker, electrical check of the printed wiring board is conducted by contacting the checker pin to predetermined position in the circuit patterns.
In case that electrical check of a double-sided printed wiring board or a multilayer printed wiring board, both having through holes, is conducted by using the above suction type checker, in order to certainly suck and fix the printed wiring board onto the checker or the table, it is necessary to blind the through holes of the printed wiring board.
When the through holes formed in the printed wiring board are blinded, the through holes are generally blinded by utilizing insulation resin films called "solder resist", because the solder resist is formed on both surfaces of the printed wiring board in the process thereof so as to protect the circuit patterns and to avoid adherence of melt solder to areas of the circuit patterns where solder should not be adhered.
Here, a conventional process for blinding the through holes of a double-sided printed wiring board will be described hereinafter with reference to FIGS. 12 and 13.
First, as shown in FIG. 12, a penetration hole 22 is formed in the double-sided printed wiring board 21 on both surfaces of which circuit patterns ( not shown ) are formed, thereafter a conductive layer 23 is plated and formed around an inner wall 22A of the penetration hole 22 and peripheral portions of both an upper and lower openings 22B, by conducting through hole plating. Thereby, a through hole 24 is formed in the printed wiring board 21 and a land 25 is formed around the through hole 24.
Further, in order to blind the through hole 24 formed according to the above, as shown in FIG. 13, a solder resist film 27 is formed on both surfaces of the printed wiring board 21 by coating a liquid composition mainly composed of photosensitive resin on the surfaces of the wiring board 21 through a curtain coating method or screen printing and thereafter by photocuring the composition. Instead of the above, the solder resist film 27 may be formed by hot-pressing a resist film composed of photosensitive resin onto both surfaces of the wiring board 21 and thereafter by photocuring thereof.
At that time, one of the solder resist films 27 is formed on one side ( the upper side in FIG. 13 ) of the printed wiring board 21 so that one opening 24A ( the upper opening 24A in FIG. 13 ) in the through hole 24 is blinded by the solder resist film 27. On the other side ( the lower side in FIG. 13 ) of the printed wiring board 21, the solder resist film 27 is formed so that the other opening 24A ( the lower opening 24A in FIG. 13 ) is not blinded.
However, in the conventional process that the solder resist film 27 is formed by coating the liquid composition of the photosensitive resin, a thickness of the liquid composition coated at corners 24B of the through hole 24 necessarily becomes very thin due to edge effect. Thus, it occurs a phenomenon that a thickness of the solder resist film 27 becomes very thin. In case that the solder resist film 27 becomes thin, cracks are apt to occur in the solder resist film 27 positioned at the corners 24B of the through hole 24. As a result, there is a problem that the solder resist film 27 is dropped from the inner wall of the through hole 24 and becomes trash in the through hole 24.
In the conventional process that the solder resist film 27 is formed by hot-pressing the resist film, it is difficult that the resist film flexibly follows to a difference in level occurring between the upper surfaces of the conductive layer 23 and the printed wiring board 21. Therefore, there is a fear that a gap (space) is formed between the solder resist film 27 and the land 25 after hot-pressing of the resist film. Thus, as the case may be, there is a problem that the solder resist film 27 is peeled from the printed wiring board 21 when the wiring board 21 is sucked at the time that electrical check is conducted by the checker. Similar to the above, since the gap is apt to be formed between the resist film and the circuit patterns due to the difference in level occurring between both the surfaces of the circuit patterns and the wiring board 21, it is not desirable in view of pattern protection. Further, the resist film is very expensive, thus the cost of the printed wiring board 21 cannot be reduced.
In both the above mentioned processes, one side of the through hole 24 is blinded by the solder resist film 27 and the other side ( opened side ) of the through hole 24 is sucked and fixed on the checker or the table of the checker when electrical check of the circuit patterns is conducted. Thus, in order to efficiently conduct electrical check, the lands for electrically checking the circuit patterns have to be formed on the upper side of the printed wiring board. As a result, there is a problem that the circuit patterns become complex.